Polyester films made of polyethylene terephthalate, polyethylene-2,6-naphthalate and the like are excellent in mechanical characteristic, heat resistance, dimensional stability, drug resistance, cost performance characteristic and so forth, and are applied in many uses by utilizing those performances. As one of the uses, a ribbon for thermal transfer can be cited.
A thermal transfer recording method that employs a ribbon for thermal transfer is excellent in cost performance, maintenance characteristic, operation characteristic and so forth and therefore are used in the fields of FAX, bar code printing and the like. In recent years, the use of color thermal transfer inks has added characteristics such as high definition and high image quality, and thus the thermal transfer recording method is used also in color thermal transfer printers and the like.
Such thermal transfer recording methods are methods in which an ink ribbon for thermal transfer in which a thermal transfer layer that contains a color material such as a pigment or a dye and so forth, and a binding agent is provided on a polyester film is superposed on an image-receiving sheet and, from the reverse side of the thermal transfer ink ribbon, heat is applied by a thermal head to melt the aforementioned thermal transfer layer and therefore fuse it onto the image-receiving sheet so that an image is formed on this image-receiving sheet.
As for the thermal transfer ink ribbon, because heat is applied thereto by a thermal head as mentioned above, the film for use in an ink ribbon for thermal transfer is required to have heat resistance. Furthermore, from the viewpoint of causing the sliding between the thermal head and the polyester film (anti-stick property) at the time of printing to be favorable to improve image quality, the film for use in the ink ribbon for thermal transfer is required to have a sliding characteristic as well. To obtain a film that has the aforementioned characteristics, it has been a common practice to provide on the aforementioned polyester film a slipping layer that is good in heat resistance and anti-stick property as a slipping layer of the thermal transfer ink ribbon which forms the opposite side to the surface on which the ink is provided.
However, the surface of the polyester film is low in wettability and therefore poor in adhesion property, and therefore has a problem that if a slipping layer in which wax or the like is a main component is directly coated on the surface of the polyester film, the slipping layer does not become closely attached. Therefore, to make the adhesion property with respect to the slipping layer firm, the film surface is subjected to an electric discharging treatment (electric discharge (EC) treatment) or the like in various gas atmospheres and the like to better the wettability of the film surface and then a slipping layer is provided. Thus, various investigations to achieve both the anti-stick property and the adhesion property between the polyester film and the slipping layer have been carried out.
For example, a proposal in which wax is further added in a substrate of a film on which wax is provided (refer to Japanese Unexamined Patent Publication bulletin (Kokai) No. H7-81018) and, furthermore, a proposal in which the coefficient of static friction and the three-dimensional roughness of a film are prescribed (refer to Japanese Unexamined Patent Publication bulletin (Kokai) No. 2004-59861), and a proposal in which the composition of the slipping layer is prescribed (refer to Japanese Unexamined Patent Publication bulletin (Kokai) No. H9-239931) are cited. Attempts to provide a slipping layer more excellent in anti-stick property have been made.
However, in recent years, a demand for high-definition and high-image quality color printing is increasing in the use for thermal transfer ribbons. For high-definition and high-image quality color printing, there is a need to perform high-level adjustment of the amount of transfer of ink by adjusting the amount of heat from the thermal head. To that end, the film for an ink ribbon for thermal transfer is required to have ink coating uniformity characteristic on the transfer layer side.
Furthermore, a film before being provided with a transfer layer is wound as a film roll and stored due to the nature of the processing steps. If a wax component of the slipping layer is provided in a large amount to improve the anti-stick property, there is a tendency that, at the time of winding the film, the wax of the slipping layer is transferred to the surface of a face of the film on which a transfer layer is to be provided, or that, when the film roll is stored for a long time, the film roll becomes wound tighter due to air pressure or temperature in the chamber and, as for the transfer thereof, denser transfer will result. Such transfer impedes a product to be used in thermal transfer use from generating high-definition and high-image quality colors because, starting at the location where transfer of wax has occurred, the transferred component repels application of ink.
Therefore, it could be helpful to provide a polyester film excellent in anti-stick property and whose slipping layer does not affect the transfer layer at the time of printing so that high-image quality printing can be performed.